Man-made reservoirs have been used from the earliest eras of civilization to store water for consumption, irrigation and control of downstream flows for flood control purposes.
Sediment transport in streams and rivers is a natural process that occurs continuously as water flows in a stream channel. The sediment flows consist of suspended particles that are conveyed in the water column due to turbulence in the water. Bed load sediment particles are conveyed along the bottom of the water channel because of the shear stress caused by the vertical velocity gradient of the water column. Sediment in streams and rivers reservoirs originates in the runoff from upstream watersheds. The concentration of the sediment in the water course is a function of the acreage, the nature of the ground cover, the average slope of the land, the degree of land disturbance, and the volume and intensity of precipitation on that watershed.
When a barrier is placed in a water course the flow of sediment downstream is blocked. The pool of water that is created upstream of the barrier causes the average water velocity to be reduced which in turn reduces the sediment carrying capacity of the flow. The rate of accumulation varies for each reservoir, but sediment always accumulates in the upstream pool which continuously reduces the storage capacity of the reservoir. When the sediment accumulates to a significant amount the original purpose of the reservoir may become impaired and ultimately retirement of the facility will be required.
Sediment deposits in reservoirs may be managed to restore enough capacity for the facility to maintain serviceability. An economic method for sediment removal that has been in use for many years employs dredging. Typically, the process includes the excavation of submerged sediment deposits with a dredge and transport of the sediment and water slurry in a pressurized pipeline to a designated disposal area. This process utilizes a floating vessel that has a movable cutterhead attached to a motorized pump that mixes excavated sediment with reservoir water creating a slurry for transport via a floating pipeline to a designated disposal area. Disposal areas have traditionally been in confined disposal areas located on either upland acreage, downstream acreage or in open water reaches of the reservoir suitable for filling.
The slurry that is directed into disposal areas produces a deposit of saturated sediment and a discharge of effluent that can be high in suspended solids. Active or passive removal of sediment solids from the process effluent is typically required to meet discharge water quality requirements. Disposal of settled sediment is subject to regulation by Local, State and Federal Authorities. Significant studies to assess the suitability of the disposal areas for use are required and considerable expense may be required to secure property that will be utilized for sediment disposal. As the more economic disposal areas are consumed with sediment storage the expense of future sediment disposal increases.
Another consequence of the construction of dams and reservoirs in the stream and river channels is the reduction in sediment flows downstream of the dam. This typically causes the downstream channel to degrade and erode. The quality of the water downstream of the dam will also change. Sediment previously transported in the water column is removed and the clarity of the water increases, which may appear to be beneficial, but, causes a change to the historic environmental conditions for aquatic vertebrates and macroinvertebrates. Over time the changes in the downstream rivers and streams become the accepted condition as popular sport fisheries proliferate in the waters more suited to clear water species.
Out of view from the surface of the impoundment, the accumulated sediment delta progresses towards the dam that is creating the reservoir. Over the service life of the reservoir the deposit ultimately reaches the face of the dam, and given enough time, will render the facility unserviceable to store water or regulate discharges. At this point, retirement, dismantling and removal of the dam may be warranted.